An ultrasound diagnostic apparatus for imaging an ultrasonogram comprises that supplying a drive signal to an ultrasound probe for transmitting ultrasound waves to a subject, receiving a reflection echo generated from the subject to the ultrasound probe, and reconstructuring and displaying the ultrasonogram on the basis of the reflection echo output from the ultrasound probe.
One of the ultrasound diagnostic apparatuses is well known to take an elasticity image indicating characteristics such as strain or hardness of a living tissue of the subject. For example, the ultrasound diagnostic apparatus captures images on time series on the living tissue upon applying pressure to the subject, measures the displacement of the living tissue with the correlation with the captured images on time series, obtains elasticity data (e.g., strain and modulus of elasticity) on the basis of the measured displacement, and reconstructs an elasticity image.
As methods for applying pressure to the subject upon measuring the displacement of the living body, there are a method for using a body motion, as a pressure source, for periodically pressurizing the organ (e.g., vasculitic pulsation), a method for pressurizing an ultrasound probe to the subject by manually pressing the ultrasound probe to the subject, and a method for pressurizing the subject by using a transducer (e.g., Patent Document JP 2000-60853 A1).
The prior arts including those disclosed in the Patent Document do not substantially consider a relationship between the direction (hereinafter, referred to as a tissue displacement direction) for actually displacing the living tissue upon applying pressure to the subject and an elasticity calculating direction (hereinafter, referred to as displacement search direction) for measuring the displacement of the living tissue. That is, the conventional displacement-search direction is fixedly set to an ultrasound transmitting and receiving surface in the vertical direction. On the other hand, the tissue displacement direction fluidly changes due to the pressurizing direction and the shape of a pressuring surface to the living tissue. Therefore, upon measuring the displacement of the living tissue, the displacement search direction does not match the tissue displacement direction. In this case, there is concern that a measurement value includes an error due to the displacement. When an elasticity image is reconstructed on the basis of the measurement value, the elasticity image cannot precisely indicate characteristics of the living tissue.